(1) Field of the Invention
The present invention relates to fixing devices installed in an image forming apparatus, such as a printer or a copier, and in particular to a technique of detecting abnormal heating in a fixing device having a resistive heating layer as a heating element.
(2) Description of the Related Art
In recent years, more and more image forming apparatuses, such as printers and copiers, employ a fixing device having, as a heating element, a resistive heating layer that generates heat by Joule heating upon passage of current. In such a fixing device, the heating element generates heat by feeding power directly to the resistive heating layer, which is effective to improve the heat utilization efficiency and to shorten the warm-up time.
The resistive heating layer is formed from insulating material, such as heat-resistant resin, and conductive material, such as metal, dispersed in the insulating material. In addition, since direct contact with the resistive heating layer may cause an electric shock, it is common to coat the resistive heating layer with an insulating layer. For example, JP patent application publication No. 2009-109997 discloses a fixing device having, as a heating element, a resistive heating layer coated with an insulating layer.
However, a typical insulating layer is as thin as the order of a few hundreds of micrometers and therefore vulnerable to scratches by contact with a foreign object or a recording sheet. If a scratch is deep enough to reach the resistive heating layer and extends in a direction not parallel to the current flow direction (especially in the direction perpendicular to the current flow direction), the current tends to collect intensively at locations around the ends of the scratch as a result of bypassing the scratch. Consequently, the current density becomes locally high at such locations of the resistive heating layer, which in turn causes heat generation to abnormally high temperatures at such locations.
Since leaving such abnormal heating may result in damaging the fixing device, it is necessary to accurately detect occurrence of abnormal heating to timely take an appropriate measure, such as shutting off the power supply to the fixing device, to avoid or minimize damage to the fixing device.
The fixing device is equipped with a temperature sensor, such as a thermistor or a thermostat. Therefore, occurrence of abnormal heating in the resistive heating layer is detected by the temperature sensor, so that a necessary measure can be taken to prevent damage to the fixing device.
Unfortunately, however, the temperature sensor, such as a thermistor or thermostat, installed in the fixing device typically has a narrow sensing range and may not be able to detect occurrence of abnormal heating, depending on the location in the resistive heating layer where abnormal heating takes place.
One method to solve the above problem is to monitor the current flowing through the resistive heating layer to detect occurrence of abnormal heating from a change in the value of an electric current between the normal operation time and the time of abnormal heating. While this method solves the above problem as the entire resistive heating layer is included in the sensing range, the amount of change in the electric current is usually small. This method is therefore susceptible to measurement errors associated with the current detection circuit and presents another problem of high probability of false-positive and false-negative results in detection of abnormal heating.